1. Field of the Invention
This invention relates to an illuminating device. More particularly, the present invention relates to an illuminating device in which a light source is provided separately from a lamp device.
2. Related Art
Japanese Patent Application Laid-Open No. 6-68702 discloses an illuminating device where light from a light source is introduced into one end of a fiberoptic cable, guided through the cable, and emitted from the other end of the cable so as to irradiate objects in front of the cable through a light-wave conducting path.
In such a device, as is illustrated in FIGS. 7A and 7B, when light emitted from a light source is introduced into illuminating device 20 after having travelled through fiberoptic cable 30, unless the light output port of the light source (not illustrated) and illuminating device 20 are disposed on the same straight line, fiberoptic cable 30 must be bent in order to have the light enter illuminating device 20. Bending the fiberoptic cable causes losses in light propagating therethrough unless the cable is bent by radius of curvature R larger than a certain level. Fiberoptic cable 30 consists of core 31 and clad 32 covering core 31, as is shown in FIG. 7A, and the index of refraction of clad 32 is smaller than that of core 31. When core 31 is formed of acrylic material and clad 32 of fluororesin, for example, the optical indices are 1.484 and 1.344, respectively.
Such a fiberoptic cable has a critical angle .theta. that will insure total reflection of incident light entering core 31. The critical angle .theta. is found to be 65.degree. using the following formula (1): EQU n sin .theta.=n' sin 90.degree. (1)
or EQU .theta.=sin.sup.-1 (n'/n)
Here, n represents the optical index of refraction of core 31 and n' represents the index of refraction of clad 32.
In order to introduce light with an angle that exceeds the critical angle, if the diameter of the fiberoptic cable having a composition as described above is 12.5 mm, the minimum radius of curvature is found to be 100 mm. Thus, in the case of FIGS. 7A and 7B, certain restrictions are placed upon the manner in which the fiberoptic cable may be arranged, and if such a fiberoptic cable is installed in a motor vehicle, problems arise and the overall size of the device may become quite large. Furthermore, if the fiberoptic cable should be forcibly bent in order to fit the cable within a limited space, the utilization of the output light is decreased.